Porous materials are commonly coloured with dyes that are applied, for example, as stains. In order to achieve satisfactory colouring results with this method a requirement is that the porous material to be coloured has a high and uniform affinity for the dye. Porous materials coloured with dyes possess, however, a light stability which is very unsatisfactory especially in outdoors applications. This is particularly a problem in the case of materials which are themselves highly weather-fast, such as metal oxides, because the colour undesirably disappears much earlier than the material is itself altered.
Another method of imparting a coloured appearance to porous materials is to provide them with a pigmented coating. This method, however, has the disadvantage that the visual appearance of porous materials such as metal oxides is changed. This is a great disadvantage, especially with metal oxides, since it is their properties, especially their transparency when applied in thin layers, that are the most prized. In addition, a pigmented coating is generally much less stable to weather and chemicals than, for example, the metal oxides onto which they are coated. With time, pigmented coatings decrease in gloss, fade and eventually become brittle and finally peel off.
U.S. Pat. No. 4,066,816 discloses the deposition of undefined inorganic pigmentary material onto at least 3 .mu.m, preferably 15-30 .mu.m thick porous aluminium oxide layers. However, very little colouration is achieved unless the pore diameter is increased, and this process is hardly useful for commercial purposes because of the predominant bronze tone (muddiness). U.S. Pat. No. 4,310,586 teaches to grow a second oxide film beneath the inorganic deposits, but this process is very difficult to control as a whole, as the colour results from optical interference and small thickness variations of the deposits or the second oxide film lead to undesirably large colour changes. Moreover, the products have less satisfactory chemical and physical properties than usual oxide layers.
U.S. Pat. No. 5,585,189 dicloses an electrophoresis process for pigmenting anodic oxide films on aluminium. However, it is necessary to increase the pore size to 50 to 250 nm, whereby the visual appearance and the weathering properties are impaired due to the rough surface. In addition, this process does not lead to satisfactory results with high performance red pigments such as diketo-pyrrolo-[3,4-c]-pyrroles and quinacridones.
Hence, the coloration of metal oxides or of materials comprising a solid substrate and a surface layer of a metal oxide has long been a problem with no satisfactory solution.
EP 648 770 and EP 648 817 disclose carbamate-functional, soluble chromophores which can be converted to the corresponding pigments by being heated to relatively high temperatures, with the ensuing elimination of the carbamate radicals. These compounds are suitable for the mass colouring of polymers and, according to EP 654 711, for the colouring of resists and of polymer coatings to which they are applied. Compounds of the same type but with improved properties are known from EP 742 556, WO 98/32802, WO 98/45757, WO 99/01511, CH-98/1922 and CH-99/702.
U.S. Pat. No. 5,243,052 discloses carbonates of quinophthalones, which are of limited solubility and can be used in heat-sensitive recording systems. The leuco dye is embedded within a polymer, preferably in polyethyloxazoline.
Soluble derivatives of triphenylmethane dyes are known from U.S. Pat. No. 4,828,976. They are likewise used in heat-sensitive recording systems, together with a binder such as cellulose acetate-butyrate, polyvinyl pyrrolidone or copolymerized ethylene/maleic anhydride.
EP 742 556, furthermore, describes textured pigmentary coatings which are prepared from soluble or meltable precursors and which cover all of or part of a substrate surface. The substrates mentioned include fibres and fabrics. It has been found, however, that these pigmentary coatings fail to entirely meet high requirements especially in terms of their rub fastness.